Transformer calculation Help

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brother

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I was looking at a transformer and the overcurrent devices doesnt appear right.Correct me if im wrong.

Its a 75 kva 3 phase 480 primary , 120/208 secondary transformer.
the breaker feeding the transformer is 125 amps. this sounds right but the secondary breaker is 400 amps. This sounds wrong.

primary protection
75,000/(480 X 1.73)= 90 amps
90 X 1.25 = 112.5 amps
next higher size breaker is 125 amps NEC 2008 240.6(A)


secondary protection
75,000/(208 X 1.73)=208.42
208.42 X 1.25=260.52 AMPS next size up is 300 amps NOT 400 amps.

This is only for the protection of the transformer and not the conductors, but doesnt the secondary sound wrong?? am I missing something from table 450.3(B) ??
 
If the primary protection is sized at not more than 125% of the FLA, the size of the secondary protective device is immaterial, as long as the conductors are protected by a tap rule.

In my opinion the primary protective device can not be "rounded" up past 125% unless there is a secondary protective device sized at 125% max (again no rounding up).
 
I agree but The next higher size is more accurately driven by note 1 to table 450.3B,

your 260A sec ocpd is ok where as the pri ocpd exceeds 125%.
 
cpal said:
What size conductors are tapped to the tranms sec and what is the total conductor length from the trans terminal to the point of sec ocp?
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I do not know the size of the conductors that are from the secondary, I would have to open up the panel to see. But im not asking about the conductor protection, Im asking about the transformer secondary protection, so that would be immaterial.
 
I suppose what iM having trouble with is WHEN secondary protection is required as im looking at the table.

I think I might have it figured out, correct me if im wrong please. looking at table 450.3(B) when you have Primary protection ONLY at 125% then there is no need for the secondary protection of the windings on the transformer,

If you have primary protection at 250% THEN secondary protection is REQUIRED at 125% . Is this correct?? Both of these are from the current of 9 amps or more columns.
 
Its a 75 kva 3 phase 480 primary , 120/208 3 phase secondary transformer.


I have a spec on my desk right now calling for this same transformer except it calls for the primary to be fed from a 150amp breaker and the secondary panel main breaker to be 225amps.
 
brother said:
I suppose what iM having trouble with is WHEN secondary protection is required as im looking at the table.

I think I might have it figured out, correct me if im wrong please. looking at table 450.3(B) when you have Primary protection ONLY at 125% then there is no need for the secondary protection of the windings on the transformer,

If you have primary protection at 250% THEN secondary protection is REQUIRED at 125% . Is this correct?? Both of these are from the current of 9 amps or more columns.
Click to expand...

Yes, you are correct.

I prefer to say: 125% primary protection is always required, secondary protection is never required. Except you may go up to 250% IF there is secondary protection. This is a 'chicken or the egg' type of discussion - but the end result is the same.
 
I agree with Jim

That 400 A ocpd is there to limit the current on the seconday tap conductors. I'm not sure why a 400 ocpd is installed.

The secondary conductors are additionally limited by 240.21 (C) thats why I inquired about the lengthof condutors
 
jim dungar said:
Yes, you are correct.

I prefer to say: 125% primary protection is always required, secondary protection is never required. Except you may go up to 250% IF there is secondary protection. This is a 'chicken or the egg' type of discussion - but the end result is the same.
Click to expand...

Let me add, this is discussion about transformer protection per 450.
Secondary conductors must be protected per 240 requirements and options.
 
3-Phase

3-Phase

Remember, this is a 3-phase delta-wye transformer. Therefore secondary protection at 125% is required, whether the primary protection is 125% or 250%, as is allowed. NEC 450.3(B)
 
beanland said:
Remember, this is a 3-phase delta-wye transformer. Therefore secondary protection at 125% is required, whether the primary protection is 125% or 250%, as is allowed. NEC 450.3(B)
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450.3(B) does not require secondary transformer protection.
The fact that the system is multi-voltage means that secondary conductor protection is required per 240.21(C) regardless the rating of the primary device.
 
Transformer Protection vs. Conductor Protection

Transformer Protection vs. Conductor Protection

Thank you for that correction. NEC does not require secondary protection at the transformer if primary 125% OCPD is used, however, this leads to the possibility of overloading of the transformer windings (internal conductors.)

A 125% OCPD rating on a 480-208Y/120V transformer under balanced loading will lead to a 217% loading for single-phase-only loading (125% x 208/120). In effect, the primary OCPD does not provide the 125% protection of the secondary windings of the transformer. 240.21(C)(1) states this clearly, for conductors, rather than windings.

But, like a service, 240.21 permits the OCPD that protects the conductors to be located remote from the transformer. This protects the conductors.

What protects the secondary windings of a delta-wye transformer from single-phase overloading?
 
the question may more accurately be (any PE's out there?) at what % of overload will the sec cond temp exceed the rating of the sec conductors??
 
beanland said:
...

A 125% OCPD rating on a 480-208Y/120V transformer under balanced loading will lead to a 217% loading for single-phase-only loading (125% x 208/120).

...
Click to expand...
Can you elaborate? I cannot envision what you are describing. An example stating secondary load conditions and how you calculate primary current on each line would help.
 
jim dungar said:
...
In my opinion the primary protective device can not be "rounded" up past 125% unless there is a secondary protective device sized at 125% max (again no rounding up).
Click to expand...
Jim,
What is the code basis for your opinion? Table 450.3(B) says that "note 1" applies to both of the 125% rules. Note 1 says we can round up to the next standard size OCPD. Only the values of 167% and up in this table cannot be rounded up to the next standard size.
 
This has always confused me as well. Is the primary conductors and OCPD
required to be sized by the KVA plus 1.25 even if there aren't any continuous
loads?
 
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